Hi,
I believe there is a lot of quality variation, from place to place. I snapped off a couple grade 8s that came from one of my local sources and am now a little skeptical about their stuff. Next time I was in the store I checked and found all their hardware boxes said "Made in China". There is probably some very good stuff coming out of China, but I have to wonder if items are graded accurately and quality control is good. Maybe it is, maybe not.
I am really leery of nuts and bolts found at the big discount stores like Home Depot, Lowes, etc. Prices might be good, but quality seems iffy and their selection wasn't all that good. The stuff from the auto parts stores is little better. At least, the pre-packaged stuff is iffy. Many auto parts stores also have bins, maybe behind the counter, of loose items that are higher quality.
The key thing to remember when buying bolts is the application. If high sheer strength is needed, go to grade 8. For example, all the front suspension bolts on my car are now grade 8, fine thread. I'd guess is Triumph originally used grade 5 there, but IMHO it doesn't hurt to be overly cautious in critical areas.
Use fine thread in high vibration areas. Coarse is best in cast metals. Also carefully consider the shank of the bolt, to keep threads from ending up inside the hole. Properly sized bolts will torque better, stay tight better and help avoid problems from vibration, such as enlarging the holes they ride in. It's possible to cut down a bolt with a Dremel cutoff wheel, if the bolt with the right shank is too long overall for the application.
I try to use some sort of locking nut or washer. In some cases, only a standard nut and lock washer is possible, due to thread length of a stud or close clearances (at the water pump, for example). A nylock is better when possible, but can only be re-used once or twice.
When rebuilding the front suspension on my car, I found the replacement parts often called for nylock nuts in low torque areas (trunnions and upper fulcrums, for example) where Triumph had originally used castellated nuts. I didn't trust nylocks in these applications, so drilled them and used cotter pins and castellated nuts.
With old suspension and body parts, I feel it's often a good idea to use a washer even where one wasn't used originally. I think it helps spread the load more evenly on the old casting or steel part, and is just a good precaution.
The crossbrace you mentioned is a pretty critical item and gets some stresses during cornering, in particular. I used 3/8" NF bolts grade 8 with shanks that prevent any threads inside the holes, nylock nuts and backed up everything with grade 8 flat washers. The bolts with the right size shanks were too long overall, so I cut them down with a Dremel cutoff wheel.
I've found some hard to find nuts and bolts at a local Ace Hardware, but their per-piece prices are absurd. I'm fortunate to have another hardware store nearby that carries a pretty good selection of hardware and have only failed to find a few things.
No one local seems to offer all-metal self-locking nuts, just nylocks at best. For drive shaft flanges I ordered a kit from TRF, which included grade 8 bolts and all-metal nuts that are an upgrade over nylocks. Even in this instance, I found the bolts a little long and running too close to the differential housing for my liking (less than 1/32" clearance! and ended up cutting about 1/8" off the end of each bolt that was just protruding from the nut anyway.
Cheers!
Alan